20S proteasome

The 20S proteasome has a barrel – shaped structure arranged as four heptomeric rings of αββα. In eukaryotes, each of α and β ring is composed of seven different proteins. The β1, β2 and β5 subunits have ‘caspase-like’, ‘trypsin-like’ and ‘chymotypsin-like’ activities, respectively. In 26S proteasome-mediated protein degradation, to entry the β chamber of the 20S proteasome that houses the proteolytic sites, a substrate protein has to pass through a substrate translocation channel consisting of the double-ring formed by six ATPases of PA700 and the α chamber formed by α subunits of the 20S proteasome.